1. Field of the Invention
This invention relates to synthetic-resin materials in the nature of a cycloaliphatic epoxy resin provided with an aluminum trihydrate filler and finely divided, highly dispersed aluminum oxide (alumina) thickener and having both excellent chemical resistance in an environment of arced gaseous sulfur hexafluoride and excellent "arc and track resistance", i.e., resistance to degradation as a result of the passage of an electric current arc along or upon surfaces of objects made of such material, in one environment of SF.sub.6 gas and especially degradation of the kind whereby there is formed on the surface of the object a relatively low-resistance track of carbon. The invention further relates to the use of such materials in the making of parts that, when used, are subjected to an environment of arced sulfur hexafluoride, such as parts on extra-high-voltage power circuit breakers. The invention also relates to articles of manufacture made of such material.
2. Description of the Prior Art
To persons skilled in the art of the transmission of electrical power at high voltages, it is known to provide the transmission lines with power circuit breakers. These are structures of substantial size, being on the order of 10-20 feet tall and having, in the vicinity of their tops, electrically insulated and supported contacts that may be rapidly opened whenever an overload or fault occurs on the transmission line being protected by such breaker. With three-phase alternating current at a voltage on the order of 750 kilovolts, it is customary to use a set of three breakers, one for each phase. When the contacts of such a power circuit breaker are opened, an arc results, and it is naturally desirable that the arc be extinguished as quickly as possible in order to avoid damage to the circuit breaker. Moreover, with voltages as high as indicated above, the arc may be several inches long, or even as long as a few feet.
It is also known that one desirable attribute of a power circuit breaker is extremely high reliability. If a power circuit breaker fails to operate, there may be serious consequences at the site of the cause of the overload, at the site where the power is being generated, at the site of the circuit breaker, or elsewhere in the system. In the design and construction of power circuit breakers known to those skilled in the art, it has not been customary to spare expense, since the cost of the circuit breaker is small in comparison with that of the power generating and transmitting equipment that it protects.
In about the last ten years, it has become customary in certain designs to provide the contact area of a power circuit breaker with a flow of sulfur hexafluoride. Sulfur hexafluoride is a gas at room temperature and atmospheric pressure, and it is chemically rather inactive. It has a dielectric value substantially higher than that of air, so that an electric arc therein not only tends to be smaller, i.e., more filamentary, but also to decay and be extinguished substantially more rapidly. However, an electric arc causes degradation of sulfur hexafluoride into chemical entities that are extremely reactive, such as positively or negatively charged fluorine atoms and the like. These chemical entities are capable of abstracting hydrogen from molecules having an O--H bond or other active hydrogens, to form hydrogen fluoride, which is extremely reactive to many insulating materials. The reactivity of arced sulfur hexafluoride is aggravated by the presence of moisture, and moisture cannot always be completely excluded from the vicinity of the contacts of a power circuit breaker.
In building power circuit breakers of the kind protected with sulfur hexafluoride, it has been customary to lead the SF.sub.6 gas from a compressor and high-pressure reservoir through a feed tube, wherein the SF.sub.6 gas is under pressure of about 250 pounds per square inch, to the vicinity of the contacts, where SF.sub.6 gas is maintained at a lower pressure such as 50 pounds per square inch. The feed tube may be visualized as a simple cylindrical tube, about 12 feet long, 3 inches in outside diameter, and 1/4 inch in wall thickness. Prior to the present invention, it has been customary to make such feed tubes by coating a sheet of paper on one or both sides with phenolic resin and rolling the paper to form the feed tube. The paper provides the strength required for containing the high-pressure SF.sub.6 gas. To obtain the required strength without internal reinforcement of the resin would require the use of impractically large wall thicknesses.
Epoxy resins of various kinds are known, including ones based upon a backbone structure comprising a pair of cycloaliphatic rings joined by a bridge comprising, for example, an ether, ester or other linkage. Although it is known that epoxy resins do, in general, have desirable properties as respects strength, dielectric constant, and resistance to chemical media ordinarily encountered, the reactivity of arced SF.sub.6 gas is so high that many other substances, considered just as unreactive in the ordinary run of chemical media encountered, have failed in an atmosphere of arced SF.sub.6 gas. For example, arced SF.sub.6 gas attacks silica, porcelain and glass. As experience with phenolic paper feed tubes in power circuit breakers demonstrates, arced SF.sub.6 also attacks phenolic resin, at least to some extent.
The development of a satisfactory feed tube for use in power circuit breakers of the kind using SF.sub.6 involves more than finding a material that is chemically resistant to the reactive entities present in arced SF.sub.6. A resinous material for this purpose must also be reasonably convenient to handle and cure, and it must also possess adequate arc and track resistance.
It is also known that the feed tubes of power circuit breakers may, in use, be subjected to considerable variations in temperature, such as from minus 30.degree. C. to 140.degree. C. Such temperature changes cause expansion and contraction, and since the resin must be strengthened, as mentioned above, by incorporating or embedding therein a strengthening member of different material, whose coefficient of expansion cannot be matched with that of the resin, there is a further requirement that the composition used exhibit satisfactory flexibility.
A further requirement of a suitable composition of matter for the above-indicated purpose is that it be sufficiently thixotropic. No known resin, unmodified, would suitably resist run-off and sagging and give the desired high and uniform build that is required in compositions for feed tubes and related purposes. Most of the known thixotropic agents, however, such as very finely divided silica, are subject to attack by arced SF.sub.6 and would be expected, therefore, to be unsuitable.
All of the above-mentioned problems were completely solved and all of the requirements met by Luck and Gainer, in the compositions taught in U.S. Pat. No. 3,828,000, which used a cycloaliphatic epoxy resin filled with aluminum trihydrate and thickened with a highly refined short fiber Coalinga asbestos.
However, the use of asbestos-containing materials has been severely curtailed in industry because of health hazards associated around possible lung injestion of asbestos particles, and because of the strict requirements now established by the Occupational Safety and Health Administration of the U.S. Government. What is needed then is a substitute material for asbestos, possessing equally unique thixotropic and SF.sub.6 resistant properties.